FIG. 1 shows a typical optical lithographic fabrication system 200 for delineating features in a wafer (substrate) 100 or in one or more layers of material(s) (not shown) located on a top major surface of the wafer, typically a semiconductor wafer (substrate). More specifically, optical radiation from an optical source 106, such as a mercury lamp, propagates through an aperture in an opaque screen 105, an optical collimating lens 104, a mask or reticle 103, and an optical focusing lens 102. The optical radiation emanating from the reticle 103 is focused by the lens 102 onto a photoresist layer 101 located on the top major surface of the wafer 100 itself or, alteratively, on the layer(s) on the top surface of the wafer 100. Thus, the pattern of the reticle 103--that is, its pattern of transparent and opaque portions--is focused on the photoresist layer 101. Depending upon whether this photoresist is positive or negative, when it is subjected to a developing process, typically a wet developer, the material of the photoresist is removed or remains at and only at areas where the optical radiation was incident. Thus, the pattern of the mask is transferred to ("printed on") the photoresist layer 101. Subsequent etching processes, such as wet etching or dry plasma etching, remove selected portions of the substrate or of the layer(s) of material(s) (not shown) located between the top major surface of the wafer and the bottom surface of the photoresist layer, or of both the substrate and the layer(s). Portions of the substrate or of the layer(s) of material thus are removed from the top surface of the wafer 100 at areas underlying where the photoresist layer 101 was removed by the developing process but not at areas underlying where the photoresist remains. Thus, the pattern of the mask 103 is transferred to the wafer 100 or to the layer(s) of material(s) overlying the wafer 100, as is desired, for example, in the art of semiconductor integrated circuit fabrication.
In fabricating such circuits, it is desirable to have as many devices, such as transistors, per wafer. Hence it is desirable to have as small a transistor or other feature size as possible, such as the feature size of a metallization stripe--i.e., its width W--or of an aperture in an insulating layer which is to be filled with metal, in order to form electrical connections, for example, between one level of metallization and another. Thus, for example, if it is desired to print the corresponding isolated feature having a width equal to W on the photoresist layer 101, a feature having a width equal to C must be located on the mask (reticle) 103. According to geometric optics, if this feature of width equal to C is a simple aperture in an opaque layer, then the ratio W/C=m, where m=L2/L1, and where m is known as the lateral magnification. When diffraction effects become important, however, the edges of the image become fuzzy (lose their sharpness); hence the resolution of the mask features when focused on the photoresist layer deteriorates.
In a paper entitled "New Phase Shifting Mask with Self-Aligned Phase Shifters for a Quarter Micron Lithography" published in International Electron Device Meeting (IEDM) Technical Digest, pp. 57-60 (3.3.1-3.3.4) (December, 1989), A. Nitayama et al. taught the use of masks having transparent phase-shifting portions in an effort to achieve improved resolution--i.e., improved sharpness of the image of the mask features focused on the photoresist layer 101. More specifically, these masks contained suitably patterned transparent optical phase-shifting layers, i.e., layers having edges located at predetermined distances from the edges of the opaque portions of the mask. Each of these phase-shifting layers had a thickness t equal to .lambda./2(n-1), where .lambda. is the wavelength of the optical radiation from the source 106 (FIG. 1) and n is the refractive index of the phase-shifting layers. Thus, as known in the art, these layers introduced phase shifts (delays) of .pi. radians in the optical radiation. By virtue of diffraction principles, the presence of these phase-shifting layers should produce the desired improved resolution. Such masks are called "phase-shifting" masks.
The mask structure described by A. Nitayama et al., op. cit., was manufactured by a single-alignment-level process involving a step of wet etching an opaque chromium layer located underneath a phase-shifting layer of PMMA which is resistant to the wet etching, whereby the etching of the chromium layer undercut the PMMA layer and formed a phase-shifting mask. However, the lateral etching of the chromium layer is difficult to control, so that the positioning of the edges of the chromium layer is likewise difficult to control. Yet this positioning of the edges of the opaque chromium must be carefully controlled in order to yield the desired improved resolution for the mask.
Moreover, the method taught by A. Nitayama et al. is not sufficiently versatile to make masks having arbitrary features, such as clustered line-space features or isolated apertures having relatively narrow-sized phase-shifting assistslots. Clustered line-space features, as well as apertures having phase-shifting slots, can be made in accordance with the teachings in a paper by M. D. Levenson et al. entitled "Improved Resolution in Photolithography with a Phase-Shifting Mask," published in IEEE Transactions on Electron Devices, Vol. ED-29, pp. 1828-1836 (1982). The technique taught therein to make such mask devices, however, required two alignment steps (two-alignment-level method), and is therefore undesirable from the standpoints of accurate alignment and decreased throughput of the mask-making procedure (typically electron beam lithography).
Isolated apertures with phase-shifting assist-slots have been taught in a paper by T. Terasawa et al. entitled "0.3 .mu.m Optical Lithography Using Phase-Shifting Mask," published in SPIE, Vol. 1088, Optical/Laser Microlithogaphy II (1989), pp. 25-33, at FIG. 2. But that method also requires a two-level alignment and is therefore likewise undesirable.
Therefore, it would be desirable to have a more versatile and more controllable single-alignment-level method of manufacturing phase-shifting masks. More generally, it would be desirable to have a more versatile and more controllable single-alignment-level lithographic technique for achieving self-aligned features.